The process design for sludge freezing must be based on the warmest winter of record to ensure reliable performance at all times. The most accurate approach is to examine the weather records for a particular location and determine how many 3-in. (8-cm) layers could be frozen each winter. The winter with the lowest total depth is then the design year. This approach might assume, for example, that the first layer is applied to the bed on November 1 each year. Equation 9.2 is rearranged and used with the weather data to determine the number of days required to freeze the layer:
With an 8-cm layer and m = 2.04, the equation becomes:
In U.S. customary units (3-in. layer, m = 0.6 in. [°F-d]-1/2):
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